1. Field of the Invention
This invention relates to a catalyst for purification of exhaust gases capable of purifying exhaust gases containing carbon monoxide (CO), hydrocarbon (HC), nitrogen oxides (NOx) and sulfur oxides (SOx) from internal combustion engines of automobiles and the like, and more particularly the catalyst also capable of preventing sulfur oxides deposited on the catalyst from being reduced by hydrogen, hydrocarbon and so on and being exhausted as hydrogen sulfide.
2. Prior Art
As a conventional catalyst for purification of exhaust gases, there has generally been known a catalyst comprising a support substrate, an active alumina coating layer formed on surface of the support substrate and catalyst ingredients loaded on the active alumina coating layer.
This catalyst for purification of exhaust gases oxidizes and decreases HC and CO contained in exhaust gases exhausted from internal combustion engines, and reduces and decreases NOx contained in the exhaust gases. Sulfur dioxide (SO.sub.2 ), generated by burning sulfur in fuel, is deposited and stored on the active alumina coating layer or a catalyst carrier layer as sulfur trioxide (SO.sub.3) by oxidation reaction in oxidizing atmosphere. Under a condition where a catalyst bed temperature is more than about 600.degree. C. and exhaust gases are rich in unburnt hydrocarbon and carbon monoxide, however, reductive hydrogen is generated by a noble metal catalyst and hydrogen-sulfide is generated by reducing the sulfur trioxide deposited on the catalyst carrier layer with the reductive hydrogen. As a result, exhaust gases having malodor is often exhausted when the fuel is a sulfur-rich fuel.
As a method for controlling the hydrogen sulfide exhaustion, an additional catalyst has been provided for reducing the hydrogen sulfide a disclosed in Japanese Examined Utility Model Publication (KOKOKU) No. 31210/1979.
Also, SAE Technical Paper Series 872134 November 2-5, 1987 discloses a 3-way catalyst controlling exhaustion of the hydrogen sulfide by reducing the hydrogen sulfide with added nickel. The 3-way catalyst is a catalyst capable of purifying CO, HC and NOx simultaneously.
However, it is not preferred to provide the additional catalyst for reducing the hydrogen sulfide because of increasing cost and weight. In another method, in which nickel is added to the 3-way catalyst, the surface area of the active alumina decreases and the catalytic activity decreases, because the active alumina loading the noble metal catalyst ingredients thereon reacts with nickel at elevated temperature, to form NiAl.sub.2 O.sub.4 having a spinel-type structure, for example.
Accordingly, it is an object of this invention to provide a catalyst having high activity for purification of exhaust gases, and being capable of suppressing the reaction of nickel with active alumina as well as being capable of suppressing the emission of the hydrogen sulfide.